Editing Nitrate (section)

== Detection in chemical analysis ==
The nitrate [[anion]] is commonly analysed in water by [[ion chromatography]] (IC) along with other anions also present in the solution. The main advantage of IC is its ease and the simultaneous [[Analytical chemistry|analysis]] of all the anions present in the aqueous sample. Since the emergence of IC instruments in the 1980s, this separation technique, coupled with many detectors, has become commonplace in the chemical analysis laboratory and is the preferred and most widely used method for nitrate and nitrite analyses.<ref name="Michalski2006">{{cite journal | last1=Michalski | first1=Rajmund | date=2006 | title=Ion chromatography as a reference method for determination of inorganic ions in water and wastewater | journal=Critical Reviews in Analytical Chemistry | volume=36 | issue=2 | issn=1040-8347 | doi=10.1080/10408340600713678 | pages=107–127}}</ref>  

Previously, nitrate determination relied on [[Spectrophotometry|spectrophotometric]] and [[Colorimetric analysis|colorimetric]] measurements after a specific reagent is added to the [[Solution (chemistry)|solution]] to reveal a characteristic color (often red because it absorbs visible light in the blue). Because of interferences with the brown color of [[Dissolved organic carbon|dissolved organic matter]] (DOM: [[Humic substance|humic]] and [[Humic substance|fulvic acids]]) often present in [[soil]] pore water, artefacts can easily affect the [[absorbance]] values. In case of weak interference, a blank measurement with only a naturally brown-colored water sample can be sufficient to subtract the undesired background from the measured sample absorbance. If the DOM brown color is too intense, the water samples must be pretreated, and inorganic nitrogen species must be separated before measurement. Meanwhile, for clear water samples, colorimetric instruments retain the advantage of being less expensive and sometimes portable, making them an affordable option for fast routine controls or field measurements. 

Colorimetric methods for the specific detection of nitrate ({{chem2|NO3-}}) often rely on its conversion to [[nitrite]] ({{chem2|NO2-}}) followed by nitrite-specific tests. The [[Redox|reduction]] of nitrate to nitrite can be effected by a [[copper]]-[[cadmium]] [[alloy]], metallic [[zinc]],<ref name="daAscencao2024">{{cite journal | last1=da Ascenção | first1=Wellington Diego | last2=Augusto | first2=Caroline Cristine | last3=de Melo | first3=Vitor Hugo Soares | last4=Batista | first4=Bruno Lemos | date=2024-05-23 | title=A simple, ecofriendly, and fast method for nitrate quantification in bottled water using visible spectrophotometry | journal=Toxics | volume=12 | issue=6 | issn=2305-6304 | pmid=38922063 | pmc=11209534 | doi=10.3390/toxics12060383 | doi-access=free | page=383}}</ref> or [[hydrazine]]. The most popular of these assays is the [[Griess test]], whereby nitrite is converted to a deeply red colored [[azo dye]] suited for [[Ultraviolet–visible spectroscopy|UV–vis spectrophotometry]] analysis. The method exploits the reactivity of [[nitrous acid]] ({{chem2|HNO2}}) derived from the acidification of nitrite. Nitrous acid selectively reacts with aromatic amines to give [[Diazonium compound|diazonium salts]], which in turn couple with a second reagent to give the [[azo dye]]. The [[detection limit]] is 0.02 to 2 μM.<ref name="Moorcroft2001">{{cite journal | vauthors = Moorcroft MJ, Davis J, Compton RG | title = Detection and determination of nitrate and nitrite: a review | journal = Talanta | volume = 54 | issue = 5 | pages = 785–803 | date = June 2001 | pmid = 18968301 | doi = 10.1016/S0039-9140(01)00323-X }}</ref> Such methods have been highly adapted to biological samples<ref name="Ellis1998">{{cite journal | vauthors = Ellis G, Adatia I, Yazdanpanah M, Makela SK | title = Nitrite and nitrate analyses: a clinical biochemistry perspective | journal = Clinical Biochemistry | volume = 31 | issue = 4 | pages = 195–220 | date = June 1998 | pmid = 9646943 | doi = 10.1016/S0009-9120(98)00015-0}}</ref> and soil samples.<ref name="Bremmer1965">{{Citation |last1=Bremner |first1=J. M. |title=Inorganic forms of nitrogen |date=1965 |work=Methods of Soil Analysis |pages=1179–1237 |url=https://acsess.onlinelibrary.wiley.com/doi/abs/10.2134/agronmonogr9.2.c33 |access-date=2025-03-03 |publisher=John Wiley & Sons, Ltd |language=en |doi=10.2134/agronmonogr9.2.c33 |isbn=978-0-89118-204-7}}</ref><ref name="Guiot1975">{{Cite journal |last=Guiot |first=J. |date=1975 |title=Estimation of soil nitrogen reserves by determination of mineral nitrogen |url=https://www.cabidigitallibrary.org/doi/full/10.5555/19751922999 |journal=Revue de l'Agriculture (Bruxelles) |volume=28 |issue=5 |pages=1117–1132 |via=CABI Databases}}</ref>

In the [[dimethylphenol]] method, 1 mL of concentrated [[sulfuric acid]] ({{chem2|H2SO4}}) is added to 200 μL of the solution being tested for nitrate. Under strongly acidic conditions, nitrate ions react with 2,6-dimethylphenol, forming a yellow compound, [[Nitrophenol|4-nitro-2,6-dimethylphenol]]. This occurs through [[electrophilic aromatic substitution]] where the intermediate [[nitronium]] ({{chem2|+NO2}}) ions attack the [[aromatic ring]] of dimethylphenol. The resulting product ([[Nitrophenol|ortho- or para-nitro-dimethylphenol]]) is analyzed using [[UV-visible spectroscopy|UV-vis spectrophotometry]] at 345 nm according to the [[Lambert-Beer law]].<ref name="dimethylphenol">{{Cite web| author=US-EPA| title=Approved method for water and wastewater analysis, 40 CFR part 136; and drinking water, 40 CFR part 141.23.| url=https://downloads.regulations.gov/EPA-HQ-OW-2011-0413-0018/content.pdf| access-date=2025-04-14}}</ref><ref name="HachTNTplus">{{Cite web| author=Hach Company| title=TNTplus 835/836 nitrate method 10206. Spectrophotometric measurement of nitrate in water and wastewater| url=https://downloads.regulations.gov/EPA-HQ-OW-2011-0413-0018/content.pdf| year=2025| access-date=2025-04-14}}.</ref>

Another [[Colorimetric analysis|colorimetric method]] based on the [[chromotropic acid]] (dihydroxynaphthalene-disulfonic acid) was also developed by West and Lyles in 1960 for the direct [[Spectrophotometry|spectrophotometric]] determination of nitrate [[Ion|anions]].<ref name="WestLyles1960">{{Cite journal |last1=West |first1=Philip W. |last2=Lyles |first2=George L. |date=1960 |title=A new method for the determination of nitrates |url=https://linkinghub.elsevier.com/retrieve/pii/S0003267060800578 |journal=Analytica Chimica Acta |volume=23 |pages=227–232 |doi=10.1016/S0003-2670(60)80057-8 |issn=0003-2670}}</ref>

If [[formic acid]] is added to a mixture of [[brucine]]  (an [[alkaloid]] related to [[strychnine]]) and [[potassium nitrate]] ({{chem2|KNO3}}), its color instantly turns red. This reaction has been used for the direct [[Colorimetric analysis|colorimetric detection]] of nitrates.<ref name="Baker1967">{{Cite web |last=Baker |first=Aaron Sidney |date=1967-05-01 |title=Colorimetric determination of nitrate in soil and plant extracts with brucine |url=https://pubs.acs.org/doi/pdf/10.1021/jf60153a004 |access-date=2025-03-01 |website=ACS Publications |language=EN |doi=10.1021/jf60153a004}}</ref>

For direct online chemical analysis using a flow-through system, the water sample is introduced by a [[peristaltic pump]] in a [[Flow injection analysis|flow injection analyzer]], and the nitrate or resulting nitrite-containing effluent is then combined with a reagent for its colorimetric detection.